Phase equilibria modelling and LASS monazite petrochronology: P-T-t constraints on the evolution of the Priest River core complex, northern Idaho

L. M. Stevens, J. A. Baldwin, J. M. Cottle, A. R.C. Kylander-Clark

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

Phase equilibria modelling, laser-ablation split-stream (LASS)-ICP-MS petrochronology and garnet trace-element geochemistry are integrated to constrain the P-T-t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K-feldspar ± rutile ± ilmenite + quartz. Interpreted P-T paths predict maximum pressures and peak metamorphic temperatures of ~9.6-10.3 kbar and ~785-790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74-54 Ma. Dates from HREE-depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE-enriched monazite constrain the timing of garnet breakdown during near-isothermal decompression at c. 60-57 Ma. Near-isothermal decompression to ~5.0-4.4 kbar was followed by cooling and further decompression. The youngest, HREE-enriched monazite records leucosome crystallization at mid-crustal levels c. 54-44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94-81 Ma), Cretaceous-Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.

Original languageEnglish
Pages (from-to)385-411
Number of pages27
JournalJournal of Metamorphic Geology
Volume33
Issue number4
DOIs
StatePublished - May 1 2015

Keywords

  • Metamorphic core complex
  • Monazite
  • Petrochronology
  • Phase equilibria
  • Priest River

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